Field of the Invention
The present invention relates to technology of generating a bias, applicable to a developing device of an image forming apparatus.
Description of the Related Art
In a developing device of an image forming apparatus, developer housed within a developer container is carried and conveyed by a developing sleeve disposed near the opening of the container, and the developer is caused to adhere to an electrostatic latent image formed on the surface of a photosensitive member. Thus, the electrostatic latent image is developed as a visible image. In this regard, a portion of the developer, carried and conveyed by the developing sleeve but not used for the development, is conveyed to the developer container via a gap between the outer circumferential surface of the developing sleeve and the inside surface of the developer container, and is collected in the developer container. In order to prevent the developer from going out of the developer container via the gap and being scattered outside the developing device, U.S. Pat. No. 5,581,336 discloses a configuration in which an electrode (prevention electrode) is provided at a position located inside the developer container and opposing the developing sleeve. This configuration prevents or suppresses the scattering of developer by applying a bias (hereinafter, “prevention bias”) to the prevention electrode and thereby applying a force on the developer in the direction toward the developing sleeve.
As the prevention bias applied to the prevention electrode is generated from a developing bias applied to the developing sleeve, it is unnecessary to provide an additional high-voltage power supply dedicated to the purpose of generating the prevention bias. FIG. 8 is a diagram showing a configuration of a prevention bias generator circuit 900 that generates a prevention bias from a developing bias. The input terminal of the prevention bias generator circuit 900 is supplied with a developing bias generated by superimposing an AC voltage on a DC voltage. Note that in the following description, the DC component of a developing bias is referred to as “developing DC bias” and the AC component of a developing bias is referred to as “developing AC bias”. A coupling capacitor 901 blocks the developing DC bias and allows the developing AC bias to pass through it. Diodes 902 and 903, a capacitor 904, and a bleeder resistor 905 constitute a voltage doubling rectifier circuit, so that a negative bias that is equal to the peak-to-peak value of the developing AC bias is output from the output terminal and serves as prevention bias Vo. Note that the output negative bias in reality is approximately 0.9 times the peak-to-peak value of the developing AC bias due to the occurrence of a forward voltage drop across the diode 902 and a voltage drop across the bleeder resistor 905.
FIG. 9 shows the relationship among the developing DC bias, the developing AC bias, and the prevention bias in the prevention bias generator circuit 900. In FIG. 9, the developing DC bias is Vd, and the peak-to-peak value of the developing AC bias is Vpp. As described above, the prevention bias generator circuit 900 outputs a negative bias that is approximately 0.9 times the value of Vpp. In other words, the absolute value of the prevention bias Vo is approximately Vpp×0.9. In order to apply a force on the developer in the direction toward the developing sleeve by the prevention bias, the prevention bias needs to be negative as is the developing DC bias, and the absolute value thereof needs to be greater than the absolute value of the developing DC bias by a predetermined amount or more. In other words, the potential difference Vh=|Vo−Vd| needs to be equal to or greater than a predetermined value. However, the developing AC bias and the developing DC bias vary depending on the environment in which the image forming apparatus is installed, the length of the period of use of the apparatus, the printing mode, etc. The prevention bias Vo output by the prevention bias generator circuit 900 decreases as the developing AC bias decreases, and therefore there is the possibility that the potential difference Vh would be smaller than the required amount.